Pressure sensor system

ABSTRACT

A pressure sensor that responds to a predetermined rate of change of pressure in the fluid medium comprises a housing that encloses a pressure chamber which has a divider wall that consists of a printed circuit board and a flexible diaphragm which is permeable. The divider wall serves to separate the pressure chamber from the pressure sensitive system in which the sensor is used. The sensor may be used in a pneumatic tire and in such use the flexible diaphragm will be deflected to activate a transmitter when the pressure in the tire changes at a hazardous rate. The sensor will not activate the transmitter when the rate of change of pressure is less than a hazardous rate of change.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 07/509,631 filed Apr. 19, 1990, now U.S. Pat. No. 5,055,826,which is a continuation-in-part of application Ser. No. 07/447,807 filedDec. 8, 1989, now U.S. Pat. No. 4,975,679, which is in turn acontinuation in part of U.S. patent application Ser. No. 07/202,262filed Jun. 6, 1988, now U.S. Pat. No. 4,890,090.

BACKGROUND OF THE INVENTION

This invention relates to a pressure sensor that responds to apredetermined rate of change of pressure in a fluid medium of a pressuresensitive system. In particular, this invention relates to a pressuresensor which may be used in a tire pressure warning system forgenerating an alarm signal when the rate of change of pressure in thetire exceeds a hazardous rate of change.

DESCRIPTION OF RELATED ART

Pressure sensitive switches that respond to pressure changes in order togenerate an alarm are well-known. The present pressure sensor, however,responds to a predetermined rate of change of pressure in a fluid mediumrather than a simple pressure differential.

U.S. Pat. No. 4,211,901 Matsuda is directed to a pressure sensing switchfor transducing the change of pneumatic pressure to an electricalsignal. This device responds to the pressure differential between twochambers that are separated by a diaphragm. The device is designed torespond to the pressure differential between the two chambers and makesno allowance for circumstances where the rate of change of pressure inthe pressure sensitive system which is being monitored is such that itdoes not represent the hazard.

U.S. Pat. No. 4,048,614 Shumway discloses the pressure detector andradio transmitter system for providing a tire pressure warning. Thissystem is designed to generate an alarm when the air pressure dropsbelow a set minimum operating pressure.

While it is important under certain circumstances to generate an alarmwhen the pressure in the pneumatic tire drops below a predeterminedcritical pressure, there are circumstances under which the merereduction in pressure in the tire below a predetermined level is nothazardous.

The wall of a pneumatic tire and the seal formed between the tire andthe rim are permeable to a certain extent and as a result the pressurein an inflated tire will fall over an extended period of time. This verygradual reduction in pressure is not hazardous when, for example, theinflated tire is mounted on a trailer of a vehicle which is held instorage for a matter of several months. It is quite common to havetrailers stored in a marshalling yard for extended periods of time andif the tires of these vehicles were fitted with tire pressure alarmsthat responded to a reduction in the pressure in the tire, these alarmswould go off frequently. Because these alarms are likely to be poweredby a low-powered battery, the batteries are likely to run down when thealarms go off and are unattended with the result that it would benecessary to replace the batteries before the device could beeffectively reactivated. This involves removing the tire from the rim.In some circumstances the fact that the alarm has gone off and thebattery has discharged may not be readily evident and this could givethe operator a false sense of security in that the operator is not awareof the fact that the device is inoperable.

In most motor vehicles a slow loss in the pressure in a tire is nothazardous. As previously indicated, all tires are permeable to someextent with the result that air is escaping on a continuous basis fromvirtually all inflated pneumatic tires. This loss in pressure onlybecomes hazardous when the rate at which it occurs is such that thepressure in the tire can drop below a critical pressure during on-roaduse.

SUMMARY OF INVENTION

It is an object of the present invention to provide a pressure sensorthat responds to a predetermined rate of change of pressure in a fluidmedium of a pressure sensitive system.

It is a further object of the present invention to provide a pressuresensor in which a pressure chamber is formed and wherein one of thewalls of the chamber is a permeable wall which serves to permit thepressure in the pressure chamber to change at a predetermined rate andwherein one of the walls of the pressure chamber is a flexible diaphragmthat moves to a predetermined position in response to a predeterminedchange in the pressure differential between the pressure chamber and thepressure sensitive system with which the sensor communicates.

It is a further object of the present invention to provide a tirepressure alarm system that incorporates a pressure sensor that respondsto a predetermined rate of change of the air pressure in the tire.

It is a still further object of the present invention to provide adivider wall in a pressure sensor that consists of a printed circuitboard and a flexible diaphram which are arranges in a face-to-facerelationship.

According to one aspect of the present invention there is provided apressure sensor switch that is responsive to a predetermined rate ofchange of pressure in a fluid medium of a pressure sensitive systemcomprising; a housing having an enclosure formed therein, a divider wallassembly comprising; a printed circuit board and a flexible diaphramarranged in a face-to-face relationship and clamped together within saidhousing and dividing said enclosure into an input chamber and a pressurechamber, the printed circuit board having a normally open alarm signalgenerating circuit thereon, circuit closing means carried by theflexible diaphram for movement, in response to a predetermined pressuredifferential between the pressure in the input chamber and the pressurein the pressure chamber, from a first position in which the circuitclosing means it is out of contact with said circuit to a secondposition in which it serves to complete the circuit to generate an alarmsignal, said divider wall assembly being permeable to said fluid mediumand having a sufficient permeability to permit the fluid medium to passtherethrough at a rate that will serve to permit equalization of thepressure in the pressure chamber and the pressure sensitive system toprevent displacement of the diaphragm to the second position when therate of change of pressure in the pressure system is less than thatwhich is indicative of a predetermined rate of change, the permeabledivider wall assembly being sufficiently impermeable to prevent thepassage of fluid medium therethrough at a rate that will serve toprevent equalization of the pressure in the pressure chamber and thepressure sensitive system at a rate that would prevent movement of thediaphragm from said first position to said second position when thepredetermined rate of change occurs in the pressure sensitive system,and input passage means opening into said input chamber.

According to a further aspect of the present invention there is provideda pressure sensor switch that is responsive to a predetermined rate ofchange of pressure in a fluid medium of a pressure sensitive systemcomprising; a housing having an enclosure formed therein and oppositelydisposed first and second walls, a printed circuit board having a ventpassage and first and second stem passages opening therethrough, analarm circuit on said board having open terminals on a first side ofsaid board proximate said vent passage, first and second valve memberseach having a stem that is proportioned to pass freely through saidfirst and second stem passages and a shoulder portion which is too largeto enter said first or second stem passages, each valve member having avalve passage that extends inwardly from a first end of the valve memberthrough the shoulder portion into the stem portion, said valve passagehaving an orifice that opens laterally through a side wall of the stemportion, a flexible diaphragm having first and second sleeve portionsintegrally formed therewith that project from opposite sides thereof,each sleeve portion having a passage opening therethrough that isproportioned to accommodate the stem portion of one of the valve membersin a close fitting relationship to close the orifice that opens throughthe side wall of the stem portion, a portion of said diaphragm beingcup-shaped and having an inner face, conductor means mounted on saidinner face of said cup-shaped portion of said diaphram, said board anddiaphram being arranged in a face-to-face relationship and mountedwithin said housing in a position in which they divide said enclosureinto an input chamber and a pressure chamber, said cup-shaped portioncooperating with the circuit board to form a collapsible chambertherebetween which communicates with the input chamber through said ventpassage, said conductor means being disposed opposite said terminalmeans so as to be movable into and out of contact therewith in responseto movement of the cup-shaped portion to activate and deactivate saidalarm circuit, input passage means opening into said input chamber foradmitting the fluid medium to said input chamber, said first valvemember being located in said enclosure with the shoulder portion thereofclamped between said first wall and the circuit board with the stemthereof extending through said first stem passage into said first sleevesuch that the orifice of the first stem is normally closed by the firstsleeve, said first sleeve being flexible such that when the pressure inthe input chamber exceeds the pressure in the pressure chamber by apredetermined amount the first sleeve will be deflected away from thefirst stem to permit the fluid medium to pass from the input chamberinto the pressure chamber, said second valve member being located insaid enclosure with the shoulder portion thereof clamped between saidsecond wall and the circuit board with the stem thereof extendingthrough said second vent passage into said second sleeve such that theorifice of the second stem is normally closed by the second sleeve, saidsecond sleeve being flexible such that when the pressure in the pressurechamber exceeds the pressure in the input chamber by a predeterminedamount the second sleeve will be deflected away from the second stem topermit the fluid medium to pass from the pressure chamber into the inputchamber, interior wall means extending from the second wall toward thecircuit board and clamping the flexible diaphram against the circuitboard, said diaphragm being permeable to said fluid medium and having asufficient permeability to permit the fluid medium to pass therethroughat a rate that will serve to permit equalization of the pressure in thepressure chamber and the pressure sensitive system to preventdisplacement of the diaphragm to the second position when the rate ofchange of pressure in the pressure system is less than that which isindicative of a predetermined rate of change, the permeable wall beingsufficiently impermeable to prevent the passage of fluid mediumtherethrough at a rate that will serve to prevent equalization of thepressure in the pressure chamber and the pressure sensitive system at arate that would prevent movement of the diaphragm from said firstposition to said second position when the predetermined rate of changeoccurs in the pressure sensitive system.

PREFERRED EMBODIMENT

The invention will be more clearly understood after reference to thefollowing detailed specification read in conjunction with the drawingswherein;

FIG. 1 is a sectional side view of a pressure sensitive electricalswitch constructed in accordance with an embodiment of the presentinvention,

FIG. 2 is a diagram illustrating a pressure sensitive transmitterassembly incorporating the pressure sensitive switch of FIG. 1,

FIG. 3 is a diagrammatic representation of a receiver for use inassociation with the transmitter of FIG. 1,

FIG. 4 is a sectional view taken through an inflated wheel assemblyillustrating the manner in which the transmitter and pressure sensitiveswitch are mounted within the inflation chamber of a tire,

FIG. 5 is a pictorial view of the mounting bracket of FIG. 4,

FIG. 6 is a partially sectioned pictorial view of a pressure sensitivetransmitter assembly,

FIG. 7 is a sectional side view through a portion of a pressuresensitive switch constructed in accordance with a further embodiment ofthe present invention,

FIG. 8 is a sectional view taken through a membrane suitable for use inassociation with the switch constructed in accordance with FIG. 7.

FIG. 9 is a sectional side view of a tire pressure arm systemconstructed in accordance with a further embodiment of the presentinvention.

FIG. 10 is a diagrammatic representation of a pressure sensorconstructed in accordance with a still further aspect of the presentinvention.

FIG. 11 is a diagrammatic representation of a pressure sensorillustrating a still further embodiment of the present invention.

FIG. 12 is a diagrammatic representation of a pressure sensorillustrating a still further aspect of the present invention.

FIG. 13 is a diagrammatic representation of a pressure sensorillustrating yet another aspect of the present invention.

FIG. 14 is a circuit diagram illustrating a temperature switch arrangedin series with the transmitter.

FIG. 15 is a sectional side view of a pressure sensor constructed inaccordance with a further embodiment of the present invention.

FIG. 16 is a pictorial view of the top surface of a divider wallsuitable for use in a pressure sensor of the type illustrated in FIG.15.

FIG. 17 is a plan view of the bottom face of the divider wall of FIG.16.

FIG. 18 is a sectional side view of a pressure relief valve constructedin accordance with a further embodiment of the present invention.

FIG. 19 is a sectional side view of a pressure sensor constructed inaccordance with yet another embodiment of the present invention.

FIG. 20 is a plan view of the P.C. Board of FIG. 19.

FIG. 21 is a sectional side view of a pressure sensor constructed inaccordance with a still further embodiment of the present invention.

FIG. 22 is a sectional side view of a pressure sensor constructed inaccordance with a still further embodiment of the present invention.

FIG. 23 is a pictorial view of the diaphram that is used in the pressuresensor of FIG. 22, and

FIG. 24 is a pictorial view of the air valve member that is used in thepressure sensor of FIG. 22.

With reference to FIG. 1 of the drawings, the reference numeral 10refers generally to a pressure sensitive electrical switch constructedin accordance with an embodiment of the present invention.

The pressure sensitive electrical switch 10 comprises a housing 12 whichis constructed from three components which are identified by thereference numerals 14, 15 and 16. The component 14 is acylindrical-shaped sleeve member which has a bore 20. The component 14is made from a material which will act as a conductor such as brass orcopper.

The component 16 is formed from an insulating material and has acylindrical side wall 26 and an end wall 28. The cylindrical side wall26 has an end face 30 which is outwardly and rearwardly inclined towardthe end wall 28. The cylindrical side wall 26 is proportioned to fit inan interference fitting relationship within the bore 20 of the component14. The component 15 is formed from an insulating material and has acylindrical side wall 24 and an end wall 18. The side wall 24 has an endface 22 which is outwardly and rearwardly inclined toward the end wall18. A flexible diaphragm 32 is formed with a perimeter mounting ring 34and a thin flexible membrane 36. The mounting ring 34 is wedged betweenthe end face 22 and the end face 30. It will be seen that the mountingring 34 will be urged into contact with the inner face of the component14 to form a good electrical contact therebetween.

The components 14, 15 and 16 and the flexible diaphragm 32 are assembledas shown in FIG. 1 such that the membrane 32 serves to separate thefirst pressure chamber 38 from the second pressure chamber 40. Thesecond pressure chamber 40 is a sealed chamber. The first pressurechamber 38 has an access passage 42 which extends through a nipple 44which is initially closed by a frangible end wall 46. The components 14,15 and 16 and the diaphragm are assembled in an environment which ispressurized to the ambient or control pressure which is required in thesecond chamber 40 in use. When the device is to be used in a tire of atruck or the like the control pressure is about 100 psi.

A first electrical conductor 48 is mounted in the end wall 28 andprojects into the first pressure chamber 38. The inner end 50 of thefirst electrical conductor 48 is arranged to spaced from the diaphragm32 and serves to form a first electrical contact.

A second electrical conductor 56 is mounted in the end wall 18 andprojects therethrough. The second electrical conductor 56 serves toprovide a second contact 58 which is arranged in a spaced relationshipwith respect to the membrane 36 when it is in its normal positionillustrated in FIG. 1. Preferably, the contacts 50 and 58 are equallyspaced from the membrane 36. It will be apparent that when a pressuredifferential is established between the chambers 38 and 40, the flexiblemembrane 36 will be deflected toward the low pressure chamber and when apredetermined pressure differential is established, the membrane 36 willmake contact with one or other of the contacts 50 or 58 to effectivelyclose the switch to permit current to flow through the component 14,membrane 36 and either of the conductors 48 or 56 depending upon whichof the conductors is in contact with the diaphragm 36.

The flexible diaphragm 32 is made from an electrically conductivesilicone rubber material such as that manufactured by Armet IndustriesCorporation of Tilsonburg, Ontario, Canada. This material is normallyconsidered to be substantially impermeable to air, however, asignificant amount of air may permeate the membrane 36 if a pressuredifferential is maintained between the chambers 38 and 40 for anextended period of time (i.e. several days). Because of the gradualequalization of pressure resulting from the permeability of the membrane36 the membrane will not be deflected to contact the conductors unlessthe rate of charge of the pressure in the pressure chamber 40 exceedsthe rate at which the fluid medium permeates the membrane 36.

By pressurizing the chambers 38 and 40 to an initial pressure which isclose to the operating pressure in the system which is to be monitored,the membrane 36 will assume the neutral position. When the switch is tobe used, the end wall 46 is removed from the nipple 44 to open thepassageway 42. The passageway 42 is then placed in communication withthe pressure system which is to be monitored so that the pressure in thesystem is applied to the first pressure chamber 38. If the pressuredifferential between the chambers 38 and 40 is greater than that withinwhich the switch is designed to remain open, the flexible membrane 36will be deflected into contact with one or other of the contacts 50 or58 as previously indicated to close the switch 10.

When the pressure sensitive switch 10 is assembled in a pressurizedenvironment, the chambers 38 and 40 are pressurized to a pressure whichis substantially equal to the normal operating pressure. Any pressuredifferential which is initially present will gradually diminish as aresult of the fact that the air will eventually permeate the membrane36. The air pressure is maintained in the chambers 38 and 40 by reasonof the end wall 46 which closes the passage 42.

As shown in FIG. 2 of the drawings, the switch 10 may be used toadvantage in a pressure sensitive transmitter assembly 60. Thetransmitter assembly 60 includes a power source in the form of anelectrical battery 62 and a transmitter 64. A pressure sensitivetransmitter assembly 60 of the type described in FIG. 2 of the drawingswhich is to be used in a tire pressure warning system may employ abattery such as one or more double-A 1.5 volt long-life batteries and atransmitter in the form of a commercially available short-wave radiotransmitter such as the type commonly used to control the opening andclosing of garage door mechanisms or the like.

A portable receiver suitable for use in association with the transmitterassembly of FIG. 2 is generally identified by the reference numeral 70in FIG. 3 of the drawings. The receiver 70 may be of any conventionalconstruction compatible with the transmitter 42. The receiver 70 has anaudible alarm 72 and a visual alarm 74. The audible alarm 72 may be inthe form of an alarm buzzer and the visual alarm may be in the form ofan LED. A cancellation button 76 is provided for interrupting the alarmcircuit of the receiver. Receivers which generate audible alarms andactivate visually detectable indicators in response to the receipt of apredetermined signal are well known and will not therefore be describedin detail. The receiver of the present invention is mounted in aportable housing 78 in which a conventional pressure gage 80 may also bemounted. The pressure gage 80 has a connecting nipple 82 of a typesuitable for forming a connection with the inflation valve of a tire orthe like. The pressure gage 80 is designed to give a reading of thepressure detected when the connecting conduit 82 is coupled to a valveof a tire or the like.

As shown in FIG. 4, the pressure sensitive transmitter assembly 60 ismounted within the inflation chamber 84 of a pneumatic tire assembly ofthe type generally identified by the reference numeral 86. In thisembodiment the transmitter assembly 60 is mounted on a bracket 88 whichis in turn mounted on the inner end 90 of the valve stem 92 which ismounted on the well rim 94.

The mounting bracket 88 is illustrated in FIG. 5 of the drawings whereinit will be seen that it is shaped to provide a seat 96 and a mountingflange 98 in which a mounting passage 100 is formed. The mountingpassage 100 is proportioned to receive the valve stem 92. The seat 96has a generally dove-tail configuration.

With reference to FIG. 6 of the drawings, it will be seen that thetransmitter assembly 60 is mounted in a housing 102 which has a baseportion 104 which has a dove-tail cross-sectional configurationcorresponding to that of the seat 88. The battery 62, transmitter 64 andpressure-sensitive switch 10 are all accommodated within the housing 102with the nipple 44 projecting through a wall of the housing 102 so thatthe through passage 42 will open into the inflation chamber 84 (FIG. 4)when the end wall 46 (FIG. 1) is removed.

A pressure sensitive switch constructed in accordance with a furtherembodiment of the present invention is illustrated in FIG. 7 of thedrawings to which reference is now made. In the embodiment illustratedin FIG. 7 of the drawing, conductors 110 and 112 are mounted in the endwall 114 in a spaced parallel relationship to provide contacts 116 and118. Similarly, conductors 120 and 122 are mounted in the side wall 124to provide terminals 126 and 128 on the opposite side of the diaphragm132. An input passage 142 extends through a nipple 144 into the firstchamber 138. The through passage 142 is normally closed by an end wall146. In this embodiment, the diaphragm 132 is deflected into contactwith the contacts 116, 118 or the contacts 126, 128 depending upon thedirection in which the diaphragm is deflected. When the diaphragm makescontact with the contacts 116, 118, current can flow from the conductor110 to the conductor 112. Similarly, when the diaphragm 132 makescontact with the contacts 126 and 128, current can flow through theconductor 120 to the conductor 122.

In a still further modification which is not shown, only one set ofcontacts 116, 118 may be provided on one side of a diaphragm so thatmovement of the diaphragm can make and break the contact between thecontacts 116 and 118 as required in use.

A diaphragm suitable for use in the embodiments illustrated in FIG. 7 ofthe drawings is shown in cross-section in FIG. 8. The diaphragm includesan annular perimeter ring 150, a flexible membrane 152 and a centralboss 154. As previously indicated, the diaphragm is made from a flexiblematerial which is capable of conducting electricity. When the annularring 152 is clamped between the shoulder 24 and end face 30 (FIG. 1), itwill be deformed to assume the generally triangular configurationillustrated in FIG. 1 and this will serve to lock the ring 150 in thehousing.

The boss 154 is particularly suitable for use in the embodimentsillustrated in FIG. 7 of the drawings wherein it is desirable to provideflat end faces 156 and 158 in order to establish contact between thecontacts 116, 118 and between the contacts 126, 128 as previouslydescribed. The boss 154 and the membrane 152 are each circular in frontview.

As previously indicated, when the pressure sensitive switch is initiallyassembled, the chambers 38 and 40 are pressurized to a pressure which isabout equal to the pressure in the environment in which the pressuresensitive switch is designed to operate in use. In a typicalinstallation such as the inflation chamber of a pneumatic tire such asthat used in the trucking industry, the chambers 38 and 40 would bepressurized to about 100 psi gage. The pressure sensitive switch can bestored in this condition for an extended period of time because littleor no load will be applied to the membrane because there is little or nopressure differential between the pressure chambers 38 and 40 and anypressure differential which may initially exist will diminish by reasonof the permeability of the membrane 36. When the pressure sensitiveswitch is to be used in the manner illustrated in FIG. 4 of thedrawings, the end wall 46 of the nipple is removed to open the passage42. As a result, the pressure in the chamber 38 will drop and themembrane 36 will be deflected to make contact with the contact 50. Thisprocedure can be used to test the transmitter to ensure that it willfunction to emit a warning signal. When the pressure sensitive switch isinstalled, the tire is then inflated and inflation will continue untilthe required pressure is established within the inflation chamber 84.This pressure will be transmitted to the pressure chamber 38 of thepressure sensitive switch and will serve to deflect the membrane 36 backto the neutral position. If the tire is over-inflated, the membrane 36will be deflected to make contact with the contact 58 and this willagain activate the transmitter to generate a warning signal. After thetire has been correctly inflated, the membrane 36 will be located in aposition which is very close to the neutral position shown in FIG. 1 ofthe drawings. If there is a pressure differential between the pressurein the inflation chamber 84 and that in the reference chamber 40, thepressure differential will be reduced in time by reason of thepermeability of the membrane 36. As previously indicated, even membraneswhich are considered to be substantially impermeable are not totallyimpermeable with the result that some transfer of air will take placeover an extended period of time resulting in the elimination of pressuredifferentials which are maintained over a long period of time. If,however, the pressure in the inflation chamber 84 decreases or increasesto an undesirable extent at a rate which is greater than the rate atwhich air can permeate the membrane 36, the membrane 36 will bedeflected to form contact with the contact 50 or 58 to activate thetransmitter which will in turn emit a warning signal which will activatethe alarm mechanisms of the receiver as previously described.

A further embodiment to the present invention is illustrated in FIG. 9on the drawings wherein reference number 160 refers generally to apressure sensor and alarm system for use in a pneumatic tire.

The pressure sensor system 160 includes a housing 162 in which apressure chamber 164 is formed. A pressure sensitive electrical switchwhich is generally identified with reference number 166 is mounted atone end of the housing 162 and serves to form a wall of the pressurechamber 164. The switch 166 consists of an end cap 168 which is formedfrom an electrically conductive material and has an end wall 170 throughwhich a plurality of passages 172 open to permit the fluid medium topass to and fro between the chamber 262 and the chamber 173. The end cap168 also has a tubular wall portion 174 which fits in a close fittingrelationship within the end of the housing 162. An electricallyinsulating collar 176 is located inwardly from the tubular wall 174. Aflexible diaphragm 178 is mounted within the switch 166 and is retainedtherein by means of a retaining wall 180. The retaining wall 180 has aplurality of passage ways 182 which open therethrough such that thefluid pressure within the pressure chamber 164 is applied to one side ofthe diaphragm 178. A transmitter and battery (not shown) may be locatedwithin the pressure chamber 164 with electrical connections being madebetween the battery, the terminals and the electrically conductiveretaining wall 180 and the electrically conductive end cap 168. It willbe seen that the collar 176 which is made from an insulating materialserves to prevent direct electrical contact between the retaining wall180 and the end cap 170. The diaphragm 178 is made from an electricalconductive material and is in contact with the retaining wall 180 whenit is in its normal position shown in FIG. 9. It will be understood thatwhen the pressure in the pressure chamber 164 exceeds the pressure inthe inflation chamber by an amount such that a predetermined pressuredifferential exists between the pressure chamber 164 and the inflationchamber of the pneumatic tire in which the device is mounted in use, thediaphragm 178 will be deflected into contact with the end wall 170 ofthe end cap 168 to complete the electrical circuit and thereby activatethe transmitter as previously described.

An end cap 260 is mounted on the housing 162 and serves to form anantechamber 262 which communicates with the diaphragm 178 through thepassage 172. A small access passage 264 opens into the antechamber 262.The access passage 264 serves to ensure that the pressure in theantechamber 262 will not increase or decrease at a rate which is asgreat as the rate at which the pressure in the inflation chamber of thetire changes in response to movement of the wheel along a rough roadsurface or the like. The addition of the end cap serves to provide a"damper" which prevents the application of "spike" pressures to thediaphragm. It will be noted, however, that the passage 264 issufficiently large to ensure that the pressure in the antechamber 262will increase at a rate substantially in excess of that required inorder to indicate the hazardous rate of change in the inflation chamber.

A second wall assembly of the pressure chamber 164 is generallyidentified by the reference numeral 184. This second wall has a firstprotuberance 186 which projects into the pressure chamber 164. Acharging passage 188 extends through a second wall 184 and openslaterally outwardly through the site of the protuberance 186. A tubularsleeve 190 which is formed from a substantially impermeable elasticmaterial extends over the protuberance 186 and serves to close theopening formed by the through passage 188. A similar protuberance 192projects outwardly from the divider wall 184 and has a discharge passage194 opening therethrough which is normally closed by a sleeve 196. Afilter 198 extends over the inner face of the second wall 184 and servesto filter the fluid medium before it may be discharged from the pressurechamber through the discharge passage 194. A similar filter 200 extendsover the outer face of the second wall 184 and serves to filter thefluid medium before it can pass through the charging passage 188.

A conventional air valve stem 202 has its inner end flange 204 seated inthe chamber 206 so that it bears against the end wall 208. Throughpassage 210 extends through the stem 202 and has a threaded outer endportion 212 which serves to accommodate a Schrader valve externalthreads 214 provided to receive the conventional end cap bearing threads216 are provided to facilitate the mounting of the valve in aconventional manner. A spacer 218 separates the flange 204 of the valvestem from the filter 192 and serves to form a further chamber 220. Itwill be understood that the pressure chamber 164, chamber 206 andchamber 220 each have a circular cross-sectional configuration or anyother desired cross-sectional configuration. In use the pressure sensorsystem 160 is mounted on the rim of a wheel with the valve stem 202projecting outwardly therefrom and the housing 162 located within theinflation chamber of the tire. A conventional one way valve is locatedin the threaded outer end 212 and the tire may then be inflated in aconventional manner. Air will then pass through the through passage 210to enter the chamber 206. The air entering the chamber 206 will bedischarged through the passages 222 into the inflation chamber of thetire. As the pressure within the pressure chamber of the tire increases,air will enter the chamber 220 through the passage 224. This air willpass through the filter 200 to the charging passage 188. Eventually thepressure in the chamber 220 will be sufficient to extend the sleeve 190to an extent sufficient to open the access passage 188 to the pressurechamber 164. Air will continue to pass through the charging passage 188until the pressure differential between the chamber 220 and the pressurechamber 164 is such that the sleeve 190 contracts to close the accesspassage. It will be noted that the sleeve 190 will act to prevent thepressure in the pressure chamber 164 rising above the pressure in thechamber 220 during the initial inflation stage and will serve tomaintain the pressure chamber 164 at a pressure which is slightly lessthan that of the inflation chamber. The pressure differential which ismaintained by the sleeve 190 is greater than that required to cause theflexible diaphragm 178 to be deflected into contact with the retainingwall 180. As a result during the initial inflation stage, the diaphragm178 will be deflected into contact with the retaining wall 180 and willbe supported by the retaining wall 180. The retaining wall 180 serves toinsure that the diaphragm 178 will not be damaged in circumstances wherethe pressure differential between the inflation chamber and the pressurechamber 164 is greater than that which might otherwise be considered tobe a hazardous differential. It will be understood that during theinitial inflation of a pneumatic tire, it is quite common to inflate thetire to a pressure which is substantially above the normal operatingpressure in order to insure that the tire is properly seated on thewheel rim. After the tire has been inflated to the installationpressure, air may then be discharged through the air valve and as aresult the pressure in the tire inflation chamber may drop well belowthe pressure in the pressure chamber 164. When this occurs, the air maythen be vented from the pressure chamber 164 through the dischargepassage 194 by deflecting the sleeve 196. This discharging will continueuntil the pressure in the inflation chamber drops below the normaloperating pressure. As a result the pressure differential between thepressure chamber 164 and the tire inflation chamber is such that thepressure in the chamber 164 exceeds the pressure in the inflationchamber by an amount which greater than that required to deflect thediaphragm 178 into contact with the end wall 170 and these will activatethe transmitter. The tire is then re-inflated to increase the pressurein the inflation chamber to the required operating pressure. As aresult, the pressure in the inflation chamber will once again exceedthat in the pressure chamber. While the pressure differential is reducedby the passage of air through the charging passage 188, the pressure inthe pressure chamber will remain slightly below that of the inflationchamber and consequently, the transmitter will be deactivated.Thereafter the permeability of the diaphragm will permit a gradualelimination of the pressure differential and the diaphragm will returnto the neutral position. As a result of this procedure, the presence ofan operable alarm device is signalled by the fact that the alarm isactivated when air is vented from the overinflated tire. The alarm is,however, deactivated when the tire is reinflated to the operatingpressure and will only be reactivated when a hazardous pressure dropoccurs in the inflation chamber.

If as a result of a fast leak, the pressure in the tire inflationchamber drops at the hazardous rate which exceeds the rate ofpermeability of the diaphragm 178. The diaphragm 178 will be deflectedinto contact with the end wall 172 to activate the transmitter aspreviously described which will in turn activate the alarm system. As aresult, an alarm signal will be generated. If, on the other hand, thepressure in the air inflation chamber of the tire increases or decreasesat a rate which is less than that which is indicative of a hazardousrate of change, the permeability of the diaphragm 178 will insure thatthe diaphragm 178 is not deflected into contact with the end wall 178,and as a result the transmitter will not be activated.

The pressure sensor system illustrated in FIG. 9 can be installed in anypneumatic tire regardless of the cold inflation pressure required in thetire. The cold inflation pressure usually ranges from 28 to 30 psi forautomobiles and 80 to 100 psi for truck tires. The charging passage 188and the discharge passage 194 and their associated sleeves 190 and 196serve to allow the pressure in the pressure chamber to be automaticallyadjusted to match the pressure in the tire and as a result, the sensordevice of the present invention could just as easily be be used in ahigh pressure or low pressure system because the pressure differentialacross the diaphragm will never exceed that permitted by the sleeves 190and 196 which act as valves that open and close the charging anddischarging passages 188 and 194.

From the foregoing, it will be apparent that the pressure sensor systemof the present invention is armed and ready to respond to a hazardouspressure change without the need to be electrically live. The systemdoes not draw power from its electrical battery until the transmitter isactivated as a result of the detection of a hazardous rate of pressurechange in the tire. As a result, the operating life of the sensor systemof the present invention may equal that of the tire in which it isinstalled in use. Obviously in circumstances where a defective tire isreplaced or repaired, it is a simple matter to replace the sensor,and/or its battery. A further modification of the present invention asillustrated in FIG. 10 of the drawings wherein the rate of change sensor230 as a pressure chamber B that is separated from the chamber A bymeans of a diaphragm 278. The end wall 232 is made permeable byproviding a small orifice 234 which opens therethrough. In this devicethe diaphragm 278 is made from an electrical conductive material and thelimit plates 280 and 282 are both electrically conductive and formterminals of a switch. When the pressure in the chamber B exceeds thepressure in the chamber A the diaphragm 278 may be deflected intocontact with the wall 280 and when the pressure in the chamber A exceedsthe pressure in the chamber B, the diaphragm may be deflected intocontact with the wall 282.

The container 233 which is diagrammatically illustrated in FIG. 10 maybe used for the purposes of calibrating the sensor switch of the presentinvention. When the capacity of the container 233 and the displacementof the plunger 234 are known, it is possible to change the pressure inthe chamber A at a predetermined rate in order to determine thepermeability of the permeable wall of the chamber B to insure that thediaphragm 278 will be displaced into contact with the wall 280 or thewall 282 when a hazardous rate of change in the pressure differentialbetween the chambers A and B occurs.

A further embodiment of the present invention associated in FIG. 11 ofthe drawings wherein the permeable wall of the pressure chamber B isprovided by forming passages 236 in an impermeable end wall 238 andapply a permeable cover plate 240 to the end wall 238. By reason of thisstructure, it is not necessary to make the diaphragm 278 from apermeable material.

In a further embodiment illustrated in FIG. 12 the housing 230 islocated in a filter housing 242 such that antechambers 244 and 246 areprovided for the chambers A and B respectively. Filter elements 248 and250 serve to filter the fluid medium before it can enter the chambers Aand B respectively.

Yet another embodiment of the present invention is illustrated in FIG.13 of the drawings wherein flexible bellows 252 and 254 enclose theantechamber 244 and 246 respectively. This structure is suitable for usein an environment where it is desirable to exclude the fluid medium inwhich the sensor is operating from the fluid medium in the pressuresensor system. It will be apparent that if the sensor system 260 of FIG.13 is located in a pressure sensitive system in which the pressure dropsthe bellows 252 and 254 will both expand. If the rate at which thepressure drops is a hazardous rate, a pressure differential will beestablished between the chamber A and the chamber B because the passage234 will not permit an equilibrium to be maintained between the chambersA and B; and as a result, the diaphragm 278 will be deflected intocontact with the support valve 280 which will, in turn, activate theelectrical system to generate an alarm. Similarly, if the pressure inthe system in which the sensor 260 is located increases at a hazardousrate, the pressure in the chamber A will increase at a greater rate thanthe pressure in the chamber B; and as a result, the diaphragm 278 willbe deflected into contact with the retainer wall 282 which will againcomplete the electrical circuit to activate an alarm.

In the further modification illustrated in FIG. 14, a temperaturesensitive switch 290 is provided in parallel with the switch 10. Theswitch 290 may be in the form of a normally open bimetallic switch whichis temperature sensitive and will close to complete the circuit andgenerate an alarm when the temperature in the fluid medium in which itis located rises above a predetermined limit. As previously indicated,the sensor 10 compensates for temperature changes in the system andwhile this has distinct advantages, there are many applications in whichit is desirable to generate an alarm when the temperature in the systemexceeds a predetermined limit. For example, when a vehicle is operatingwith dual tires, one of the two tires of a set may be overloaded and mayheat up until it explodes. The system illustrated in FIG. 14 willgenerate an alarm even if the pressure sensor is not activated by thechange in pressure in the system.

As previously indicated, the pressure sensor of the present inventionresponds to the rate of change of pressure in the system which itmonitors and is not affected by the actual pressure in the system. Inaddition, the pressure sensor is not affected by temperature changes inthe system which it monitors because the sensor is housed within thesystem and temperature changes in the fluid which is being monitoredwill result in corresponding temperature changes in the sensor. Thisfeature is particularly desirable when the sensor is used in a pneumatictire where the temperature may vary substantially due to ambienttemperature conditions or speed, load and road surface conditions. Whenthe pressure sensor is mounted within the inflation chamber of a tire,pressure changes due to temperature changes will be substantially thesame in the pressure chamber and in the inflation chamber and will notactivate the alarm. In other systems where the tire pressure is comparedto outside atmospheric pressure the changes in pressure resulting fromtemperature changes could cause the system to generate a false alarm. Inthese systems a drop in ambient temperature could also generate a falsealarm.

A further embodiment of the present invention is illustrated in FIGS.15, 16 and 17 of the drawings to which reference is now made. Withreference to FIG. 15 of the drawings, reference numeral 300 refersgenerally to a pressure sensor constructed in accordance with thefurther embodiment of the present invention. The pressure sensor 300consists of a housing which is generally identified by the referencenumeral 302 and consists of a bottom cover 304 and a top cover 306. Aledge 308 is formed around the open end of the bottom cover 304. Agasket 310 is seated on the ledge 308 and a divider wall 312 rests onthe gasket 310. The top cover 306 has a lip portion 314 arranged to bearagainst the divider wall 312 to clamp the divider wall against thegasket 308.

The divider wall 312 serves to divide the interior space of theassembled housing into a pressure chamber 316 and an input or chargingchamber 318. The top cover 306 has a plurality of air input passages 320opening therethrough and a filter 322 as arranged to overlie thepassages 320 such that any air that enters the input chamber 318 isfiltered.

As shown more clearly in FIGS. 16 and 17, the divider wall 312 isrectangular in shape as is the housing 302. The divider wall 312 has anupper face 324 and a lower face 326. Major passages 328, 330, and 332open through the divider wall 312 as do minor passages 334, 335, 336,and 338.

As shown in FIG. 15 of the drawing, the stem 340 is mounted in thepassage 328 and has a main body portion 342 which projects into theinput chamber 318. A vent chamber 344 is formed in the stem 342 andopens into the pressure chamber 316. An annular recess 346 which isformed on the exterior of the circular stem 342 and serves to form anannular channel 348. Annular shoulders 350 are formed on stem 342 atopposite ends of the channel 348. Vent passages 352 open radially fromthe vent chamber 344 into the channel 348. A tubular sleeve 354 at itsopposite ends supported by the shoulders 350 of the stem and the sleeveextends over the channel 348 and serves to close off the channel 348.The tubular sleeve 354 is made from a resiliently flexible electricallyconductive material. The tubular sleeve 354 is proportioned so that theopposite ends thereof must be stretched in order to fit over theshoulders 350 with the result that in the relaxed configuration the mainbody of the sleeve will assume the concave curvature illustrated in FIG.15. It will be noted, however, that if the pressure in the channel 348is increased, the sleeve may be caused to balloon outwardly andultimately the pressure in the channel 348 may be relieved by separationof the sleeve from one or both of the shoulders 350 to permit the fluidmedium to be discharged from the channel 348 into the chamber 318.

The stem 340 and tubular sleeve 354 are both electrical conductors. Thesleeve 354 forms one terminal of an electrical switch. The otherterminal of the electrical switch is formed by a conductive bridgemember 360 which has legs 362 and 364 which extend through the passages335 and 334 (FIG. 17) of the divider wall. The main body portion 366 ofthe conductive bridge is in the form of a cylindrical member which hasan annular flange 368 which protects radially inwardly and has a passage370 opening therethrough. The inner face 372 of the passage 370 formsthe second terminal of a switch. The switch is closed when theconductive sleeve 354 is inflated to the extent that it will expandoutwardly into contact with the face 342 of the conductive bridge. Asshown in FIG. 18 of the drawings, the printed circuit has paths 374 and376 that communicate with the conductive bridge 360 and the stem 340respectively.

The conductive sleeve 354 is made from a permeable material of the typepreviously described with reference to the permeable diaphragm of theearlier embodiments such that the fluid medium that is contained in thepressure chamber 316 may pass through the tubular sleeve at a rate whichwill serve to permit equilization of the pressure on opposite sides ofthe diaphragm at a rate that would prevent detection of a change inpressure in the pressure sensitive system which occurs at a rate that isless than a hazardous rate. The permeable sleeve is sufficientlyimpermeable to prevent the passage of the fluid medium therethrough at arate which will serve to prevent equalization of the pressure onopposite sides of the sleeve at a rate which would prevent movement ofthe sleeve from the retracted position shown in FIG. 15 to the extendedposition when a hazardous rate of change occurs in the pressuresensitive system. From the foregoing it will be seen that the tubularsleeve of this embodiment performs the function of the flexiblediaphragm and the first pressure relief valve of the embodimentillustrated in FIG. 9. In addition, it will be seen that the P.C. board36 performs the function of the divider wall.

To supplement the permeability of the sleeve 354, a permeable plug 380is mounted in the passage 332. This permeable plug 380 has the samepermeability characteristics as those previously described withreference to the sleeve 354 and may be used either to supplement thepermeability of the sleeve or in circumstances where the sleeve 354 ismade from an impermeable material.

An input stem 390 is mounted in the passage 330 and has an input chamber392 which opens into the input chamber 318. Passages 394 open radiallyoutwardly from the chamber 392. A tubular sleeve 396 is mounted on thestem 390 and serves to normally close the passages 394 so that it willfunction as a charging device similar to that previously described withreference to FIG. 9.

As shown in FIG. 16 of the drawings, a chip 398 and a transmitter 400are mounted on the circuit board and are connected to the printedcircuit through the passages 336 and 338 respectively.

In use when a hazardous reduction in pressure occurs in the chamber 318,the pressure differential across the sleeve 354 will cause the sleeve354 to balloon outwardly to contact the conductive bridge and this willcomplete the circuit of the printed circuit board which will in turnactivate the transmitter 400 which will transmit a signal to the alarmdevice which will activate the alarm.

FIG. 18 of the drawings illustrates a modified stem and tubular sleeveconstruction. In the embodiment illustrated in FIG. 18, two tubularsleeves are employed. The inner tubular sleeve 500 is a highly flexiblenon-conductive sleeve and the outer sleeve 502 is a conductive sleeve.The proximal end 504 of the inner sleeve is seated in a groove 506 whichis formed in the stem 508. The proximal end of the outer sleeve 502rests on the shoulder 510 of the stem and makes electrical contact withthe stem through the shoulder 510. By using this two part sleevestructure it is possible to use a relatively thin walled conductivesleeve which will have a substantial degree of flexibility whilemaintaining its electrical conductivity. By using a highly flexiblenon-conductive sleeve 500 it is possible to ensure that the passages 512can be closed efficiently in use. This structure can be used in theembodiment illustrated in FIG. 15 when a permeable plug 380 is used.

An alternative structure is illustrated in FIGS. 19 and 20 of thedrawings. As shown in FIG. 19, the stems 520 may be of identicalconstruction and may be made from an extruded plastic material. Thestems 520 do not have to be electrically conductive and similarly theflexible sleeves 522 are not made from electrically conductive material.As a result, the stem structure is inexpensive to manufacture and simpleto install.

The diaphragm 524 is made from a conductive material and is secured tothe P.C. Board by a conductive casing 526. A conductive bridge 528 ismounted on the underside of the P.C. Board 312 and extends in a spacedrelationship with respect to the wall 530 of the casing 526. Theconductive diaphragm 526 is sufficiently flexible to be deflected to theposition shown in FIG. 19 when a predetermined pressure differential isestablished in order to complete the circuit of the P.C. Board and toactivate an alarm as previously described.

A still further embodiment of the present invention is illustrated inFIG. 21 of the drawings. In this embodiment, a pressure transducer 600is mounted in the passage 602 which opens through the P.C. Board 604.The pressure transducer is in the form of a transducer crystal which ismade permeable by forming one or more passages that open therethrough.The passages may be formed by a laser so as to provide the degree ofpermeability previously described in the other embodiments of theinvention which will ensure that the pressure transducer crystalresponds to the rate of change of pressure in the pressure sensitivesystem in a like manner to that previously described. Alternatively, theP.C. Board may be made permeable by forming one or more passages openingtherethrough by means of a laser in order to provide the requiredpermeability.

A further modification of the present invention is illustrated in FIGS.22, 23 and 24 to which reference is now made.

With reference to FIG. 22 of the drawings, the reference numeral 700refers generally to a pressure sensor constructed in accordance with yetanother embodiment of the present invention.

The pressure sensor 700 consists of a housing generally identified bythe reference numeral 702 which has a bottom cover 704 and a top cover706. A ledge 708 is formed around the open end of the bottom cover 704.A flexible diaphragm 710 and a printed circuit board 712 each have theirperipheral edge portions clamped between the ledge 708 and the loweredge 714 of the cover 706. A printed circuit board may be constructedsubstantially in accordance with the circuit board described in FIGS. 16and 17 in that it will support a chip and a transmitted and a printedcircuit that serve to connect the system to an electrical battery. Theprinted circuit board 712 differs from the circuit board of FIGS. 16 and17 in the location of the vent passage 716 and the stem passages 718 and719.

The flexible diaphragm 710 has first and second sleeve portions 730 and732 formed integrally therewith which project from opposite sidesthereof. A cup-shaped portion 734 extends from the underside of theflexible diaphragm 710. Support ribs 718 and 720 project from the bottomwall of the lower cover 704 and serve to surround the cup shaped portion736 and the sleeve portion 730 of the diaphragm 710. The main pressurechamber 736 which is formed within the lower cover 710 communicates withthe pressure chamber 724 through a passage 722. Similarly, the pressurechamber 736 communicates with the chamber 728 through a vent passage726.

First and second valve members 740 and 742 are of identical constructionand, as shown in FIG. 24, each has a stem portion 744 and a shoulderportion 746. Recess 748 is formed in the free end of the shoulderportion 746 and bent notches 750 open through the sidewall of the recess748. A valve passage 752 (FIG. 22) extends inwardly from the recess 748through the shoulder portion 742 into the stem portion 744. An orific754 opens through the sidewall of the stem 744.

A filter 756 is located in the recess 748 of the first valve member 740.Input passages 758 open through the wall of the top cover 706 such thatair may be admitted to the stem passage 752 and the input chamber 760. Acollar 762 projects from the top wall of the cover 706 and surrounds theupper end of the shoulder portion 740.

The stem portion 744 of the first valve member extends through the stempassage 718 in the printed circuit board into the sleeve 730. The sleeve730 serves to normally close the orific 754. Similarly, the stem portion744 of the second valve member 742 projects through the stem passage 719into the second sleeve 732 and the second sleeve 732 serve to normallyclose the orific 754.

The cup-shaped portion 734 supports a conductive rubber pad 764 on theinner face thereof which is arranged in a face-to-face relationship withrespect to the terminals 766 and 768 of the printed circuit that iscarried by the printed circuit board.

The flexible diaphragm 710 is permeable to the fluid medium, which is tobe admitted to the chamber 760 in use, to a sufficient extent to permitthe fluid medium to pass therethrough at a rate that will serve topermit equalization of the pressure in the pressure chamber 736 and theinput chamber 760 to prevent displacement of the cup-shaped portion ofthe diaphragm to a sufficient extent to cause the conductor 764 tocontact the term 766 and 768, when the rate of change in the pressure inthe input chamber 760 is less than that which is indicative of apredetermined hazardous rate of change. The diaphragm 710 is, however,sufficiently inpermeable to prevent the passage of fluid mediumtherethrough at a rate that will serve to prevent equalization of thepressure in the pressure chamber 736 and input chamber 760 at a ratethat would prevent movement of the diaphragm from its first position inwhich the conductor 760 is spaced from the terminal 766, 768 to a secondposition in which the conductor 764 serves to connect the terminals 766and 768 when the hazzardous rate of change in pressure occurs.

The pressure sensor switch of FIG. 22 is designed to be mounted withinthe pressure chamber of a pneumatic tire such that when the pneumatictire is inflated air will pass through the input passages 758 and filter756 into the input chamber 760. Because of the pressure differentialbetween the pressure of the incoming air that is admitted to the valvepassage 752 of the first valve member 740 the sleeve 730 will bedeflected away from the stem 744 to permit air to enter the chamber 728through orific 754. The air that is admitted to the chamber 728 willpass into the pressure chamber 736 through the orific 726 and will passinto the chamber 724 through the orific 722. In order to test thepressure sensor switch, the tire in which the switch is located shouldbe over-inflated. As a result, the pressure in the pressure chamber 736and in the input chamber 760 will exceed the normal operating pressurein the tire. After the tire has been inflated the orific 754 will beclosed by the sleeve 730 when the pressure in the pressure chamber 736is only slightly less than the pressure in the input chamber 760 by anamount of about 3 p.s.i. The cup-shaped portion 736 of the flexiblediaphragm has a sufficient flexibility to insure that if the pressure inthe input chamber 760 is lower than the pressure in the pressure chamber736 by an amount that is greater than 1 p.s.i., the diaphragm will bedeflected to cause the conductor 764 to move into contact with the terms766 and 768 to complete the alarm circuit. The cup-shape portion of thediaphragm and the conductor 764 are constructed in a manner similar tothat used to provide pressure sensitive buttons on the keyboard ofelectronic calculators and the like and this structure is well known inthe electronics industry and will not, therefore, be described ingreater detail.

After the tire has been inflated to a pressure above the normaloperating pressure and the pressure sensor has been charged to obtain anequilibrium between the pressure in the pressure chamber 736 and theinput chamber 760, the air is then charged through the inflation valveof the tire at a rate that is sufficient to generate a hazzardous rateof change in the pressure within the tire. As a result, the pressure inthe input chamber 760 will drop rapidly at a hazzardous rate. Because ofthis rapid change in pressure in the input chamber 76 the pressure inthe collapsible chamber 735 will be reduced as air is withdrawn throughthe vent passage 716 and the cup-shaped portion will collapse inwardlyto cause the conductor 764 to engage the contact 766 and 768 to therebyactivate the alarm circuit which will, in turn, generate an alarmsignal. As a result of this procedure, the operator is able to determinethat the alarm system is operating correctly. If the alarm signal is notactivated, the tire should be deflated and the sensor device should beremoved and replaced with a replacement device and the testing procedureshould then be repeated. If the alarm system is activated as a result ofthis test, the tire should continue to be deflated until the pressurewithin the tire reaches the normal operating pressure or it may bereinflated to the operational pressure if the pressure is allowed todrop below the operating pressure in the testing cycle.

It will be noted that if, during the deflation of the tire, the pressurein the chamber 760 drops below the pressure in the chamber 736 by anamount that is greater than about 3 p.s.i. the air that is trappedwithin the chamber 736 will be vented through the second valve member742 because the sleeve 732 will deflect to allow the air to escape tothe orific 754.

The structure of the pressure sensor device that is described andillustrated in FIGS. 22, 23 and 24 has a number of practical advantagesover the structures previously described. In particular, the printedcircuit board can be manufactured in a substantially conventional mannerand by combining the printed circuit board and the flexible diaphragmmember it is possible to form a divider wall assembly that effectivelyseals the input chamber from the pressure chamber. By forming the gasketas a one piece structure it is possible to reduce the likelihood ofleakage occuring at the joints between the various components of thegasket. By manufacturing the valve members 740 and 742 from plasticsmaterial and making them identical to one another, the difficultiespreviously associated with attempting to tune a transmitter are greatlyreduced. In addition, by mounting the valve members so that they areclamped between the divider walls and the oppositely disposed wall ofthe upper and and lower covers the assembly operation is alsosimplified. Furthermore, by forming the sleeve memers as an integralpart of the flexible diaphragm, it becomes a simple operation to insertthe stem portions into the bore that is formed within each sleeve. Thisassembly is much simply than attempting to thread the stem portion intoa small sleeve.

In use, when the pressure within the tire in which the pressure sensordevice is located is reduced at a hazzardous rate, the pressure in thechamber 760 will fall rapidly and will establish a substantial pressuredifferential between the chambers 736 and 760 and as a result theflexible diaphragm will be deflected as previously described to positionthe conductor 764 in contact with the terminals 766 and 768 to compeltethe alarm circuit thereby to generate an alarm signal that istransmitted from the sensor to an alarm device in the manner previouslydescribed with respect to the earlier embodiments.

In circumstances where the pressure in the chamber 760 falls at a ratethat is less than a hazzardous rate the permeability of the flexiblediaphragm will serve to maintain a sufficient equilibrium between thepressure in the input chamber and the pressure in the pressure chamberto prevent sufficient movement of the flexible diaphragm to locate theconductor 76 in contact with the terminals 766 and 768. In this respect,the assembly of the present embodiment operates in the manner describedin the prior embodiments.

Because the electrical circuit is normally open the drain on the batterywill be virtually zero. Only when a hazzardous leak is detected will thecircuit be activated.

It will be noted that as the temperature within a tire increases ordecreases due to external conditions, the pressure in the tire will riseor fall. This may represent a substantial pressure changer particularlyin truck tires. However, in view of the fact that the pressure sensor ofthe present invention is located within the tire, the entire assemblywill be subjected to these temperature changes and as a resultsubstantially the same rate of temperature change will be experienced bythe atmosphere within the chambers 760 and 736. As a result, the changein pressure within the sensor or the results from the change intemperature within the tire will be substantially the same in thechamber 760 and 736 and will not be sufficient to cause the sensordevice to be activated to indicate the hazzardous rate of change in theinput chamber 760.

A temperature sensitive electrical circuit could be incorporated in theprinted circuit board of the present invention that will cause the alarmsystem to generate an alarm signal when the temperature within the tirerises to an extent that is likely to render the switch mechanisminoperative. It is anticipated, however, that the sensor of the presentinvention will be capable of operating at temperatures of the order of75° to 80° C. which is well above the safe operating temperature for atire.

Various further modifications of the present invention over and abovethose described herein will be apparent to those skilled in the art.

I claim:
 1. A pressure sensor switch that is responsive to apredetermined rate of change of pressure in a fluid medium of a pressuresensitive system comprising;(a) a housing having an enclosure formedtherein, (b) a divider wall assembly comprising; a printed circuit boardand a flexible diaphram arranged in a face-to-face relationship andclamped together within said housing and dividing said enclosure into aninput chamber and a pressure chamber, the printed circuit board having anormally open alarm signal generating circuit thereon, circuit closingmeans carried by the flexible diaphram for movement, in response to apredetermined pressure differential between the pressure in the inputchamber and the pressure in the pressure chamber, from a first positionin which the circuit closing means it is out of contact with saidcircuit to a second position in which it serves to complete the circuitto generate an alarm signal, (c) said divider wall assembly beingpermeable to said fluid medium and having a sufficient permeability topermit the fluid medium to pass therethrough at a rate that will serveto permit equalization of the pressure in the pressure chamber and thepressure sensitive system to prevent displacement of the diaphragm tothe second position when the rate of change of pressure in the pressuresystem is less than that which is indicative of a predetermined rate ofchange, the permeable divider wall assembly being sufficientlyimpermeable to prevent the passage of fluid medium therethrough at arate that will serve to prevent equalization of the pressure in thepressure chamber and the pressure sensitive system at a rate that wouldprevent movement of the diaphragm from said first position to saidsecond position when the predetermined rate of change occurs in thepressure sensitive system, and (d) input passage means opening into saidinput chamber.
 2. A pressure sensor as claimed in claim 1 wherein saiddiaphram is shaped to provide a cup-shaped portion that cooperates withthe circuit board to form a collapsible chamber therebetween whichcommunicates with the input chamber through first vent passage thatopens through the board.
 3. A pressure sensor as claimed in claim 2wherein said circuit has spaced terminals located proximate said firstvent passage and wherein said circuit closing means comprises conductormeans which are disposed opposite said terminal means so as to bemovable into and out of contact therewith in response to movement of thecup-shaped portion to complete and interupt said alarm circuit.
 4. Apressure sensor as claimed in claim 1 wherein said printed circuit boardhas first and second and valve stem passages opening therethrough andfirst and second valve members having a stem extending freely throughsaid first and second valve passages respectively and a shoulder portionwhich is too large to enter said first or second valve stem passages,each valve member having a valve passage that extends inwardly from afirst end of the valve member through the shoulder portion into the stemportion, said valve passage having an orifice that opens laterallythrough a side wall of the stem portion.
 5. A pressure sensor as claimedin claim 4 wherein said flexible diaphragm has a main body portion andfirst and second sleeve portions integrally formed therewith thatproject from opposite sides of the main body through the first andsecond stem passages of said board and surround the first and secondstems respectively to normally close the orifice that opens through theside wall of the stem portion that is located therein, said first andsecond sleeves being flexible such that when the pressure in the inputchamber exceeds the pressure in the pressure chamber by a predeterminedamount the first sleeve will be deflected away from the first stem topermit the fluid medium to pass from the input chamber into the pressurechamber and when the pressure in the pressure chamber exceeds thepressure in the input chamber by a predetermined amount the secondsleeve will be deflected away from the second stem to permit the fluidmedium to pass from the pressure chamber into the input chamber.
 6. Apressure sensor as claimed in claim 5 wherein housing has oppositelydisposed first and second walls and said flexible diaphram is locatedopposite said second wall and wherein interior wall means extends fromthe second wall toward the circuit board and serves to clamp theflexible diaphram against the circuit board.
 7. A pressure sensor switchthat is responsive to a predetermined rate of change of pressure in afluid medium of a pressure sensitive system comprising;(a) a housinghaving an enclosure formed therein and oppositely disposed first andsecond walls, (b) a printed circuit board having a vent passage andfirst and second stem passages opening therethrough, an alarm circuit onsaid board having open terminals on a first side of said board proximatesaid vent passage, (c) first and second valve members each having a stemthat is proportioned to pass freely through said first and second stempassages and a shoulder portion which is too large to enter said firstor second stem passages, each valve member having a valve passage thatextends inwardly from a first end of the valve member through theshoulder portion into the stem portion, said valve passage having anorifice that opens laterally through a side wall of the stem portion,(d) a flexible diaphragm having first and second sleeve portionsintegrally formed therewith that project from opposite sides thereof,each sleeve portion having a passage opening therethrough that isproportioned to accommodate the stem portion of one of the valve membersin a close fitting relationship to close the orifice that opens throughthe side wall of the stem portion, a portion of said diaphragm beingcup-shaped and having an inner face, (e) conductor means mounted on saidinner face of said cup-shaped portion of said diaphram, (f) said boardand diaphram being arranged in a face-to-face relationship and mountedwithin said housing in a position in which they divide said enclosureinto an input chamber and a pressure chamber, said cup-shaped portioncooperating with the circuit board to form a collapsible chambertherebetween which communicates with the input chamber through said ventpassage, said conductor means being disposed opposite said terminalmeans so as to be movable into and out of contact therewith in responseto movement of the cup-shaped portion to activate and deactivate saidalarm circuit, (g) input passage means opening into said input chamberfor admitting the fluid medium to said input chamber, (h) said firstvalve member being located in said enclosure with the shoulder portionthereof clamped between said first wall and the circuit board with thestem thereof extending through said first stem passage into said firstsleeve such that the orifice of the first stem is normally closed by thefirst sleeve, said first sleeve being flexible such that when thepressure in the input chamber exceeds the pressure in the pressurechamber by a predetermined amount the first sleeve will be deflectedaway from the first stem to permit the fluid medium to pass from theinput chamber into the pressure chamber, (i) said second valve memberbeing located in said enclosure with the shoulder portion thereofclamped between said second wall and the circuit board with the stemthereof extending through said second vent passage into said secondsleeve such that the orifice of the second stem is normally closed bythe second sleeve, said second sleeve being flexible such that when thepressure in the pressure chamber exceeds the pressure in the inputchamber by a predetermined amount the second sleeve will be deflectedaway from the second stem to permit the fluid medium to pass from thepressure chamber into the input chamber, (j) interior wall meansextending from the second wall toward the circuit board and clamping theflexible diaphram against the circuit board, (k) said diaphragm beingpermeable to said fluid medium and having a sufficient permeability topermit the fluid medium to pass therethrough at a rate that will serveto permit equalization of the pressure in the pressure chamber and thepressure sensitive system to prevent displacement of the diaphragm tothe second position when the rate of change of pressure in the pressuresystem is less than that which is indicative of a predetermined rate ofchange, the permeable wall being sufficiently impermeable to prevent thepassage of fluid medium therethrough at a rate that will serve toprevent equalization of the pressure in the pressure chamber and thepressure sensitive system at a rate that would prevent movement of thediaphragm from said first position to said second position when thepredetermined rate of change occurs in the pressure sensitive system.